Limit SearchWP results to a single category

gistfile1.php

<?php

function searchwp_include_only_category( $ids, $engine, $terms ) {
  // Limit to category 10
  $categoryID = 10;

  // Retrieve the IDs of all the posts in this category
  $args = array( 
    'category' => $categoryID, 
    'fields'   => 'ids', 
    'nopaging' => true,
  );
  
  $ids = get_posts( $args );
  
  return empty( $ids ) ? array( 0 ) : $ids;
}
 
add_filter( 'searchwp_include', 'searchwp_include_only_category', 10, 3 );